The present invention relates to the field of data communications. Merely by way of example, the present invention is illustrated as a method and apparatus for transmitting a voice channel and a digital a 4-wire digital data service (DDS) signal over a single twisted pair telephone line.
Techniques for the transmission of multiple voice or data signals are well known in the telecommunications industry. Conventional digital data services (DDSs) over a 4-wire local loop include digital services at transmission rates of 2.4 kilo-bits/sec. (kbps), 4.8 kbps, 9.6 kbps, 19.2 kbps, and 56 kbps. Business subscribers often desire the use of such conventional DDSs for electronic data processing equipment such as telephones, computers, facsimile machines, and the like. However, it often is difficult, costly, and time consuming to add a 4-wire local loop onto a customer premises, thereby making it difficult to the obtain DDS.
For example, the conventional DDS needs to transmit digital data over "clean" telephone lines, that is, telephone lines that substantially have no abrupt changes in impedance values from wire gauge changes, bridge taps, coils, and the like. In particular, telephone companies often find it necessary to condition pre-existing telephone lines to remove such wire gauge changes, bridge taps, coils, and the like before implementing the conventional DDS onto the pre-existing telephone lines. In fact, telephone companies refer to this type of conditioning as a class C3 data conditioning technique. The C3 data conditioning technique often takes up to two weeks or even more for a typical 10-12 kilo-feet 4-wire local loop.
Another limitation with the use of the conventional 4-wire DDS is the need for repeaters. Some telephone companies even install a repeater at customer premises if line losses cause the signal level to fall below a pre-determined level such as 32 dBs using 28 kHz, 48 kHz, and 80 kHz test signals. The use of the repeater at the customer premises often occurs because certain data service unit/channel service units (DSU/CSU) operate ineffectively at low signal levels. The conventional DDS also relies upon a typical bipolar signal at a transmission rate of up to 56 kbps which often cannot flow effectively over a line span of 10-12 kilo-feet without the use of a repeater. The use of such repeater increases the cost of the conventional DDS for transmission lengths greater than 10-12 kilo-feet, and also decreases the efficiency and reliability of the conventional DDS by introducing additional elements into the telephone line span.
Still further, the convention 4-wire DDS service lacks capability for analog phone services. In particular, the 4-wire DDS service provides digital services at the aforementioned transmission rates which cannot easily convert to a plain analog phone signal representing plain old telephone services (POTS). It is often desired to have POTS at a customer premises as a "back-up" service when the conventional 4-wire DDS service fails. In addition, the conventional 4-wire DDS relies upon the use of copper from 4-wires, which is a limited telephone company resource.
From the above, it is seen that a method and apparatus for providing an increase in telephone services from a telephone company central office to a customer premises over pre-existing wires is often desired.